ballast

A ballast is a device installed in series with a gas discharge lamp to limit and regulate the current flowing through it. Without a ballast, the lamp would draw excessive current and fail almost instantly. The ballast maintains a steady operating point by introducing impedance into the circuit, preventing the runaway current that discharge lamps naturally exhibit once their arc has struck.

Fluorescent tubes, high-intensity discharge (HID) lamps, and neon signs all require ballasts. Magnetic ballasts, the traditional type, use a transformer core with copper windings and sometimes an iron shunt to produce the needed impedance. Electronic ballasts use solid-state circuits, typically operating at high frequency (20 kHz to 40 kHz), and are smaller, more efficient, and generate less heat than their magnetic counterparts. Some ballasts also incorporate a starter circuit to provide the initial high voltage needed to ionize the gas and strike the arc.

The electrical characteristic that makes a ballast necessary is the negative resistance of a gas discharge lamp. As voltage across the lamp increases, current through it increases sharply, which creates more voltage drop and current surge. A ballast with positive impedance opposes this behavior, creating a stable equilibrium. Magnetic ballasts work at line frequency (50 Hz or 60 Hz), while electronic ballasts can switch at much higher frequencies, allowing for smaller core sizes and lower reactive losses.

Ballast selection depends on lamp type, wattage, and operating voltage. A 40-watt fluorescent tube requires a different ballast than a 400-watt metal halide lamp. Mismatch between ballast and lamp leads to dimming, flicker, short lamp life, or failure to start. Magnetic ballasts can hum audibly and dissipate significant heat as copper losses; electronic ballasts are quieter and more efficient but require compatible lamps and may not work with dimmers unless specifically rated for it.

The term ballast comes from its analogous function to ballast in a ship, which stabilizes the vessel by counteracting external forces. In electrical circuits, the ballast counteracts the destabilizing tendency of discharge lamps. Most jurisdictions now regulate or phase out magnetic ballasts in favor of electronic ones due to energy efficiency requirements and the elimination of polychlorinated biphenyls (PCBs) that were formerly used as coolants in some magnetic ballasts.